With a $30$ percent probability, the flow unit coming out of the third resource has to be reworked. In that case, the operations at the first, second, and third resources are repeated. You can assume that $($a$)$ rework always succeeds $($i$.$e$.,$ a unit going through the rework loop will always work after the third resource$)$ and $($b$)$ the processing times for units in rework are the same as for regular units.

$$ The fourth resource has a processing time of $4$ minutes per unit.

a$.$Where in the process is the bottleneck?